Recently, secondary batteries, which can be charged and discharged, have been widely used as energy sources for wireless mobile devices, auxiliary power devices or the like. Secondary batteries have also attracted considerable attention as power sources for electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (plug-in HEV) or the like which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles that use fossil fuels.
Since secondary batteries are used in a variety of mobile or portable devices and the usable time thereof is limited, it is very important to determine correct information regarding the SOC of the secondary battery. The SOC is very important information for the user to use the devices since the SOC serves as a measure of how long the secondary battery can be used. Devices such as a laptop, a mobile phone or a vehicle, in which a secondary battery is typically mounted, estimate the SOC of the secondary battery, determine information such as the usable time or capacity of the secondary battery from the estimated SOC, and provide the determined information to the user.
The SOC of a secondary battery is generally expressed as a percentage of remaining capacity relative to the full charge capacity (FCC) of the secondary battery. Various methods may be used to estimate the SOC of the secondary battery and one typical method is to estimate the SOC using current integration. In the current integration method, input/output current of a secondary battery is integrated and the SOC of the battery is determined by subtracting the integral from the initial capacity.
Although the current integration method derives a relatively accurate SOC value in initial cycles, the accuracy of the current integration method may decrease as the charge cycle count of the secondary battery increases due to the occurrence of an SOC drop phenomenon in which the SOC is rapidly lowered near the end of discharge. Thus, the longer the secondary battery is used, the more rapidly the SOC drops near the end of discharge. If the current integration method is directly used in such situations, it is not possible to correctly estimate the SOC near the end of discharge due to the SOC drop phenomenon. As a result, although the SOC has been more rapidly decreased than expected, reducing the usable time of the secondary battery, the user may be unaware of the reduced usable time and fail to properly cope with the full discharge state of the secondary battery. This greatly inconveniences the user.
Thus, there is a great need to provide a method and system for estimating the remaining capacity (SOC) of a secondary battery, which can fundamentally solve such problems.